First and foremost, the primary contraception action of the oral contraceptive pill is that they prevent ovulation of an egg from the ovary. The oral contraceptive pills also change the nature of the cervical mucus especially during the fertile period by making it more hostile to sperm survival and motility. Finally, the oral contraceptive pills thin the endometrial lining making it harder for an embryo to implant. The oral contraceptive pills are very effective (as long as you are taking them correctly as instructed) and are associated with a low failure rate.
If you are older than 35 years and smoke, have a history of blood clots or breast cancer, it may be advisable that you not to take the pill as the risk of deep venous thrombosis is higher.